is basically the roof of the train station of the airport, which is located slightly away from the main terminal building, at the threshold for runway 23L/R. It is very clean, and you can get extremely close to the aircraft as seen here:

The other observation deck is ontop of the terminal building, ontop of terminal B. From here, photos opportunities are good, especially for planes landing 05L/R, and taking off 23L/R. The following photos were taken from there:

We used to have in PMI a terrace located in the main building during the 70's but somebody (very very clever) thought it was nothing necessary when the new airport was built...No windows, no open places or terraces where to take pics.
Now comes to mi mind the Mexico DF airport with huge stairs..

Another point is the short distance you are from airplanes. I don't know why in my airport we are so far to the rwys..Do they worry about if we throw stones??
I hear you, Enrique. At FRA you're close to approaching planes, but not close to the runways. The airport is surrounded by passenger & cargo terminals, Lufthansa maintenance & office buildings, 2 freeways, 20-metre-high walls (so that airport employees don't get distracted by the car noise), heavy woods & an air base. Not the best conditions for aviation photography if you ask me. Plus, you almost always have a taxiway between you & the runway.

Basically, there are just 3 spots: the terrace, the 25L approach & the Rwy 18 platform. Here are 2 pics from the vicinity of the 25L spot:

And here's the Rwy 18 spot:

I'm not a big fan of all those spots. Instead of a terrace a few dirt piles just beside the fence would do much more to better the photography situation.

From the left side of the road you can take pics without ladder ( The rwy is located on the right hand side ). Sometimes what you photogragh is a firefighter throwing water to a checkpoint from the airport!!